In recent years, ever denser optical disc devices, like CD (Compact Disc), DVD (Digital Versatile Disc) and Blu-ray, have been developed and put to practical use. Further, in a Blu-ray optical disc device, a reading speed and a writing speed have been enhanced. With the speed enhancement, there is a demand for speed enhancement of a PDIC (Photodiode Integrated Circuit) that is incorporated in an optical pickup. Because the speed enhancement comes with high-power laser output, the speed enhancement of the PDIC needs to be achieved also under high optical input. Further, because silicon has a high absorption coefficient for blue light, carriers generated by the light absorption are concentrated on the silicon surface. Therefore, the probability of occurrence of pair annihilation of photo-generated carriers due to surface recombination increases, and it is also an important issue to obtain the efficiency (photocurrent/incident light power).
A light receiving element (PD: photodiode) that is incorporated in an optical pickup is typically monolithically integrated on an IC (Integrated Circuit) for smaller size and lower cost. Therefore, constraints are placed on the optimizing design of the PD for the above-described issue. FIG. 7 shows a PDIC illustrated in FIG. 1 of Patent Literature 1, for example.
The PDIC in FIG. 7 includes a p-type silicon substrate 1, a p+-type silicon layer 2, a p−-type epitaxial layer 3, and an n-type epitaxial layer 6, and, in the course of epitaxial growth, a p-type buried diffusion layer 4 and an n-type buried diffusion layer 5 are formed. Further, a p-type separating diffusion layer 7 is formed for element separation between a PD and a bipolar transistor, and a PD part n-type diffusion layer 8 is formed in the PD region, and an n-type diffusion layer 9, a p-type base diffusion layer 10, and an n-type emitter diffusion layer 11 are formed in the bipolar transistor region. Above such a silicon substrate, a dielectric film 12 and an electrode 13 are formed.
In this PDIC, the PD and the bipolar transistor having the n-type semiconductor layer on their surfaces are monolithically integrated on the p-type silicon substrate 1. The n-type epitaxial layer 6 needs to have a thickness of about 1 μm or more for a bipolar transistor structure (cf. e.g. Patent Literature 2). In such a structure, the p-n junction position of the PD is as deep as 1 μm or more below the silicon surface, which leads to the reduction of the efficiency and the deterioration of the speed of response. The cause of the deterioration is that the absorption coefficient of silicon is large for blue light that is used in the Blu-ray optical disc device, and a depth at which the incident light intensity is 1/e is as shallow as about 0.15 μm.
In light of such concerns, PDICs described in Patent Literatures 3 to 5 adopt a technique that etches the PD region to make the p-n junction position shallower. However, an unplanarized wafer is not practical because the semiconductor process becomes difficult. Patent Literatures 6 and 7 adopt a technique that inverts the lower layer part of the n-type epitaxial layer 6 into p type by diffusion to make the p-n junction position shallower. However, it is difficult to control the concentration by a balance of p and n. The p−-type epitaxial layer 3 has a concentration of as low as about 1×1014 cm−3, for example, in order to keep the capacitance of the PD low. It is extremely difficult to invert the n-type semiconductor layer into p type by diffusion and control it to such a low concentration.
Therefore, the optimization of a PD is typically made on the assumption that the p-n junction position is deep to some extent in consistent with a bipolar transistor. In Patent Literature 2, the thickness of the n-type epitaxial layer is 2 μm, and a high concentration region is provided so that a peak is at a depth of 0.3 to 0.7 μm. However, hole carriers that are generated on the surface side relative to the concentration peak are difficult to move to the p-type region, and the probability of pair annihilation due to surface recombination increases. The efficiency is thereby low.
In Patent Literature 8, the thickness of the n-type epitaxial layer is about 0.8 μm to 1.0 μm, and an n-type impurity concentration by ion implantation is defined to thereby achieve the quantum efficiency of 90% or higher. In this manner, the n-type impurity concentration distribution of the PD part is important.
The PD part n-type diffusion layer 8 contributes to preventing hole carriers generated by light absorption from surface recombination and improving the efficiency as described in Patent Literature 9. Further, the sheet resistance of the n-layer on the PD surface affects the speed of response of the PD as described in Patent Literature 8. The presence of the PD part n-type diffusion layer 8 is effective in terms also of decreasing resistance. The PD part n-type diffusion layer 8 may have a double ion-implantation structure as described in Patent Literature 10. If the concentration profile has a gradient, an electric field is applied to photo-generated hole carriers, and the speed of response can be enhanced.
From Patent Literatures 1, 8 and 11, the quantum efficiency of 90% or higher can be achieved by setting the n-type doping profile of the PD part n-type diffusion layer 8 as follows. When arsenic is used for ion implantation and the diffusion depth is shallow, the maximum surface concentration is set to 1×1020 cm−3 or less. When phosphorus is used for ion implantation and the diffusion depth is deep, the maximum surface concentration is set to 1×1019 cm−3 or less. Further, a good speed of response can be obtained by setting the thickness of the n-type epitaxial layer 6 (p-n junction depth) to as thin as about 0.8 to 1.0 μm. In the 3 dB bandwidth of a single PD, 500 MHz or higher can be obtained. If such a high-speed response is achieved, it becomes compatible with a 12× speed Blu-ray optical disc device.